1. Field of the Invention
This invention relates generally to Floating Production Storage and Offloading vessels (FPSO's) where a turret is rotatably supported on the vessel and where the turret is fixed to the seabed by anchor legs so that the vessel can weathervane about the turret. In particular, this invention relates to stern offloading of a permanently moored LNG (Liquefied Natural Gas) liquefaction process vessel wherein pipelines on the seafloor bring natural gas to the permanently moored vessel. The gas is conducted from the seafloor to the vessel through flexible pipes (risers). An LNG plant on board the permanently moored vessel liquefies the gas, then large quantities of liquid gas are stored on board. Periodically, LNG carriers (shuttle tankers) arrive and load the liquefied gas for transport to distant ports. Temporary mooring connection and offloading to an LNG carrier vessel in offshore sea conditions requires highly reliable and safe equipment. Historically, most LNG loading operations are performed in mild environments in protected waters and at dockside. Performing this operation offshore between floating vessels is not known to have been done to date.
2. Description of the Prior Art
European patent application EP0947464A1 describes a fluid transfer system comprising a counterweighted transfer boom with coaxial fluid ducts mounted on the stern of a LNG storage vessel. The LNG storage vessel is also known by the term LNG process vessel and also LNG/FPSO. The drawings of this application show an LNG shuttle tanker (or “carrier”) moored to the LNG storage vessel by means of a hawser. A disadvantage of this system concerns the method of mooring the LNG shuttle tanker with a flexible hawser to the LNG storage vessel. The relatively short reach of the fluid transfer boom makes it necessary to bring the LNG shuttle tanker very close to the storage vessel. This creates danger of collision of the two vessels, because the hawser does not restrict the forward motion of the LNG carrier. Therefore, a tugboat is required at all times at the LNG carrier stern to attempt prevention of collision.
FIGS. 1 and 2 of the drawings illustrate a prior art LNG transfer system 3, such as the FMC Technologies BTT system, with piping and flexible joint swivels connecting the stern S of LNG/FPSO vessel 1 to the bow B of LNG carrier 2. Hawser 80 endures the mooring force to hold LNG carrier 2 to the stern S of LNG/FPSO vessel 1. The hawser mooring system of FIGS. 1 and 2 can be disadvantageous in rough seas in that there is no restraint to prevent LNG carrier 2 from surging forward and colliding with LNG/FPSO vessel 1. Piping pantograph 5 is flexible and allows limited lateral motion of pantograph fluid connector 6 (see motion arrows “L” in FIG. 2), such as within a circle of 12 meters radius. As LNG carrier 2 sways laterally, crane boom 4 rotates automatically to follow the wide excursions of the bow B of LNG carrier 2 while connected to LNG carrier manifold 7. The system of FIGS. 1 and 2 requires a large separation distance, such as 60 to 100 meters to reduce the chance of collision of the vessels. The large separation distance requires a long and expensive crane boom 4.
U.S. Pat. No. 4,568,295 shows a yoke mooring system between a buoy and a moored vessel.
3. Identification of Objects of the Invention
A primary object of the present invention is to provide a disconnectable mooring device in combination with a fluid transfer arrangement to connect an LNG carrier (shuttle tanker) to an LNG process vessel (LNG/FPSO) that is intended for frequent connection and disconnection of the LNG carrier vessel in an offshore environment of at least Hs 2 meters significant wave height that causes relative motion between the two vessels.
Another object of the invention is to provide a disconnectable mooring system that temporarily moors an LNG carrier to an LNG process and storage vessel.